1. Field of the Invention
The present invention relates to a backlight which uses a light-emitting diode or the like as a light source and which is used in display devices, and to a liquid crystal display device provided with the backlight.
2. Description of the Related Art
Conventionally, cold cathode fluorescent lamps are principally used as the light source for a backlight of a liquid crystal display device, but a cold cathode fluorescent lamp requires a dedicated lighting circuit that requires the use of high voltage. Therefore, an LED (Light Emitting Diode) is used as a light source in mobile phones, PDAs (Personal Digital Assistant), and other small applications in particular. However, the light emission of the LED is mainly an artificial white light that is obtained by coating a blue LED with a YAG fluorophor, and there are difficulties in color reproducibility, and the reproducibility of red colors in particular. It is impossible to improve this structure if it is treated as a unit, and a separate red LED must therefore be supplemented or an LED must be used that emits a red (R), green (G), or blue (B) monochromatic light. However, not only do LEDs have difficulty with color reproducibility, but also the quantity of light is low, the cost is higher than a cold cathode fluorescent lamp, and there are other problems. Therefore, there are few opportunities to use LEDs in monitors and TV applications.
On the other hand, LED efficiency has recently continued to improve, LEDs that can accommodate considerable electric currents are being developed, and, with the consideration of environmental issues, work is actively being carried out with the aim of commercializing relatively large backlights in which LEDs are used. Examples of a backlight in which such an LED is used includes one in which LEDs are provided with a plurality of light guide plates and disposed as light sources on the end face of each of the light guide plates (refer to FIG. 1A of Japanese Domestic Re-publication No. 2003-532273), and one in which a plurality of substrates having side-emitting LEDs that are disposed in an array are disposed in a box-shaped reflector, and in which this configuration is used as a surface light source.
FIG. 1 is a cross-sectional diagram schematically showing the structure of the conventional backlight described in SID 03 DIGEST, 2003, p1259-1261. The conventional backlight 100 described in SID 03 DIGEST, 2003, p1259-1261, as shown in FIG. 1, has a configuration in which a plurality of LEDs 101 that emit a red (R), green (G), or blue (B) monochromatic light are aligned in a suitable array. The direction of travel of the light emitted from the LED 101 is changed by roughly 90° at a first reflector 105, and the light enters the light guide plate 102. The monochromatic light of the colors RGB that enters the light guide plate 102 is propagated inside the light guide plate 102 repeatedly undergoing total reflection. In this interval, the colors become mixed and the light is emitted from the opposite side of the plane of incidence. The direction of travel of light emitted from the light guide plate 102 is changed 180° at a second reflector 104, and the light enters a light guide plate 103. The light that is incident to the light guide plate 103 is uniformly emitted from the top surface of the light guide plate 103 through the use of a scattering and reflective pattern disposed on the reverse surface thereof (the surface on the reflective sheet 106 side).
When the white light LEDs described above are used as the light source in such a conventional backlight, emphasis must be placed on making brightness more uniform, and there is little need for concern with regard to nonuniformity of colors, but when monochromatic RGB LEDs are used, it is not an exaggeration to say the value of the device as a backlight is determined by how efficiently the colors of the light emitted from the LEDs can be mixed. In view of the above, in order to prevent brightness and color from being nonuniform, the backlight proposed in Japanese Laid-Open Patent Application No. 2004-158336 is configured with a second reflector 104 composed of polymethyl methacrylate, polycarbonate, or another transparent body whose index of refraction is higher than that of air, and also composed of a reflective plate disposed on the external surface of the transparent body.
However, the prior art described above has the following problems. In the specific case of the backlight 100 shown in FIG. 1, a light guide plate 103 is required to cover the illuminated surface, and a light guide plate 102 is required to mix the colors of RGB monochromatic LED light. Therefore, there is a problem in that the mass of the light guide plate becomes several times greater than an ordinary edge-type backlight in which a cold cathode fluorescent lamp or the like is used and in which the light of tube is caused to directly enter the light guide plate, and the overall weight is increased. The backlight 100 also has a problem in that there are incident and excident planes of the light guide plate 102, the incident plane of the light guide plate 103, and many other interfaces between air and physical objects, causing optical loss to occur at the interfaces and the efficiency of the light to decrease.
The backlight of Japanese Laid-Open Patent Application No. 2004-158336 has the effect of improving brightness nonuniformity and color nonuniformity, but there is a problem in that there are conversely a large number of components and increased external dimensions, particularly thickness.